1. Field of the Invention
This invention relates generally to a thin film assembly, and a method of fabricating such a thin film assembly, which exhibits electrostrictive and/or piezoelectric properties. The invention relates more specifically to a method of fabricating electrostrictive and piezoelectric thin film assemblies which provides excellent molecular-level uniformity and precise structural control.
2. Description of Related Art
Electrostrictive and piezoelectric materials have attracted great attention recently for use in sensors, microelectromechanical system (xe2x80x9cMEMSxe2x80x9d) devices, and actuators. Electrostrictive materials produce displacement in reaction to an applied electric field. That is, when an electric field is applied, molecular-level polarization may change the dimensions of the material.
Piezoelectric materials, conversely, produce an electrical charge displacement when mechanically strained. This charge displacement is associated with an electric field applied over a specific distance, and thus results in an electrical potential difference. The strain-induced charge displacement results from a reorganization of molecules within the material when subjected to strain, and the creation or enhancement of the noncentrosymmetric nature of the material.
Most thin film materials used to fabricate MEMS devices and actuators are at least several microns thick, and are typically formed using conventional bulk processing methods such as casting, sol-gel technology, or spin coating. Piezoelectric and electrostrictive materials are conventionally formed through a variety of such established processes, including the strong electric field poling of ceramics and polymers containing molecular dipoles and the growth of single crystal materials from molecular precursors. For example, commercially available films such as polyvinylidenefluoride (xe2x80x9cPVDFxe2x80x9d) and its co-polymer vinylidene fluoride (xe2x80x9cVDFxe2x80x9d) and trifluoroethylene (xe2x80x9cTrFExe2x80x9d), as well as lead zirconate titanate (xe2x80x9cPZTxe2x80x9d) films, need to be poled using a high external electric field to orient the internal molecular structure. There is presently no method that can be used to form such active films directly without such poling.
Aside from the necessity of poling, however, difficulties with these conventional methods include the lack of ability to effect complete molecular orientation and the possibility of dipole orientation relaxation due to thermal, mechanical, and other effects. There is, therefore, the likelihood that defects will be formed in the material structure during growth. These difficulties are especially significant in the formation of thin film piezoelectric materials in which the effect of a few defects or misoriented dipoles can cause proportionally large changes in the performance and degradation characteristics of the piezoelectric material.
Furthermore, it is difficult to form ultrathin films with thicknesses of less than one micron, and especially films of thicknesses of less than 100 nm, by these conventional methods.
The development and design of MEMS devices and conformal actuator mechanisms therefore requires both a flexible manufacturing process that allows the formation of active thin film materials at the molecular level, and control over internal material structure.
Another processing method, the electrostatic self-assembly (xe2x80x9cESAxe2x80x9d) method, has been widely used to synthesize multifunctional high performance materials and devices. This low-cost process offers a number of advantages over conventional film synthesis techniques to form layer-by-layer composite films with excellent molecular-level uniformity and precise structural control. For example, U.S. Pat. No. 5,208,111 describes one- or multi-layered elements applied to supports which are produced by applying the individual layers from solutions of organic materials in suitable solvents to modified supports by sequential physisorption (salt formation).
U.S. Pat. No. 6,020,175 also employs the ESA method, and describes the fabrication of thin films which include multiple layers of functional molecules (such as enzymes and other proteins, pigments and dyes) admixed with polymer ions in combination with multiple layers of polymer ions without the functional molecules.
As indicated above, it is difficult to form electrostrictive and piezoelectric ultrathin films with thicknesses of less than one micron, and especially of thicknesses of less than 100 nm, by conventional methods. While the aforementioned patents disclose the use of the ESA method for the fabrication of such thin films, the use of the ESA method for the fabrication of electrostrictive and piezoelectric thin films has been heretofore unknown.
Therefore, a general need exists to provide a method of fabricating a thin film assembly which exhibits electrostrictive and/or piezoelectric properties. A more specific need exists for a method of fabricating electrostrictive and piezoelectric thin film assemblies which not only provides a thinner film than is attainable by conventional methods, but which provides excellent molecular-level uniformity and precise structural control.
It is an object of the present invention to provide a thin film assembly, and a method of fabricating such a thin film assembly, which exhibits electrostrictive and/or piezoelectric properties. It is a further object of the present invention to provide a method of fabricating electrostrictive and piezoelectric thin film assemblies which not only provides a thinner film than is attainable by conventional methods, but which provides excellent molecular-level uniformity and precise structural control.
Accordingly, the present invention advantageously relates to electrostrictive and piezoelectric thin film assemblies comprising (a) a substrate, and (b) a film comprising one or a plurality of layers disposed upon the substrate, wherein at least one of the layers comprises a dipolar material, and this layer of dipolar material has a uniform thickness of less than 500 nm, typically less than 200 nm, for example, about 0.1 nm to about 100 nm. The assembly may also include other layers having a uniform thickness which can range from about 0.1 nm to about 1000 nm, more typically about 0.1 nm to about 500 nm, and most typically about 0.1 nm to about 200 nm.
In an optional embodiment of the assembly, the assembly may further comprise an optional interlayer between the substrate and the film. The interlayer may provide adhesion, or provide a buffer layer that as a result of the differences in the thermal coefficients of expansion between the substrate and the film, prevents cracking or dislocation. Typical interlayer materials include, for example, a neutral layer applied by a conventional method, or a charged layer applied by the ESA method.
The invention further relates to a method of fabricating electrostrictive and piezoelectric thin film assemblies comprising (i) a substrate and (ii) a film comprising one or a plurality of layers disposed upon the substrate, wherein at least one of the layers comprises a dipolar material and is at most 500 nm thick. The method comprises (a) depositing on the substrate a first layer comprising ions having a same charge, and optionally, (b) depositing on the first layer in a sequential manner one or a plurality of subsequent layers, wherein each subsequent layer of the one or a plurality of subsequent layers comprises ions having a same charge which is opposite to the charge of an immediately preceding layer, wherein at least one of i) the first layer and ii) the one or a plurality of subsequent layers comprises a dipolar material and is at most 500 nm thick.
In an optional embodiment of the method of fabricating the assembly, the method further comprises before step (a), providing the substrate with ions or ionizable compounds having a same charge over the entire surface of the substrate, wherein the subsequent layer provided in step (a) comprises ions having a same charge which is opposite to the charge of the ions or ionizable compounds of the substrate.
The advantages associated with the present electrostrictive and piezoelectric thin film assemblies and method of fabrication therefor are numerous. By virtue of employing the ESA method in conjunction with at least one layer of dipolar material, the invention overcomes several problems and limitations of conventional ionic polymer materials.
First, it allows the formation of a uniform host film through the ESA process. Layer-by-layer uniformity is achieved through the inherent forming of molecular level monolayers, each of which is a thermodynamically stable state of the system. Conventional ionic polymers are formed by a casting process, with no control over physical microstructure, and little control over exact physical dimensions. The avoidance of defects and impurities in ESA-formed thin films leads to more uniform transport properties, and thus enhanced electrostrictive and/or piezoelectric response.
Secondly, a wide variety of materials including polymers, inorganic nanoclusters, and metal nanoclusters may be incorporated into the ESA films, with their microcomposite structure advantageously used to control transport properties. By varying the species and size and morphology of molecules and clusters in monolayers through the thickness of the film, the desired spatial gradient of the diffusion properties may be achieved.
Thirdly, once the ions are included in the film, protective coating outer layers with poor ionic conductivity may be formed as encapsulants in order to prevent ion escape.
Fourthly, the method may be used to form a highly conductive thin film electrode on the electrostrictive and piezoelectric film for use in devices such as actuators, sensors and MEM""s elements.
The present invention, therefore, facilitates the preparation of a large variety of electrostrictive and piezoelectric films with different coefficients and applications, for different working environments, fabricated on substrates having a variety of materials and shapes. The film thickness can be increased by increasing the self-assembly multilayers, and thus match the requirements of various devices.
The resultant electrostrictive and piezoelectric thin films utilize ionic transport to yield large, effective piezoelectric coefficients. The ESA-processed films have a layer-by-layer laminated structure, and exhibit piezoelectric response directly without electric field poling. The primary advantages of the present method over conventional material formation methods include 1) the ability to incorporate a wide variety of materials into the film; 2) the ability to form a film of uniform thickness; 3) the ability to form thermodynamically stable, molecularly oriented thin film multilayer structures at room temperature, that are not easily deoriented due to thermal and mechanical effects; and 4) the ability to form films with few, if any, of the structural defects and impurities associated with conventional methods, which heretofore have led to premature device degradation, breakdown, and failure.